This invention relates to treating a semiconductor wafer with a plasma or a glow discharge for dissociating a gas into reactive ionic and neutral species and, in particular, to a downstream etch using gas mixtures containing oxygen.
Plasma etch apparatus can be divided into two broad categories, downstream or remote etching and direct etching. In downstream etching, the semiconductor wafer is not immersed in the glow discharge, as it is in direct etching. The result is a somewhat more gentle treatment of the wafer because high frequency at high power is not coupled through the wafer. In either type of apparatus, it is known in the art to employ an oxygen bearing plasma for removing organic compounds such as photoresist or for oxidizing a semiconductor wafer.
More typically, oxygen is used in combination with other gases due to the highly reactive nature of oxygen. It is known in the art to add nitrogen or water vapor to oxygen in a downstream plasma cleaning chamber. A problem with these additives is that the energy necessary to dissociate a nitrogen molecule or a water molecule is much less than that necessary to dissociate an oxygen molecule. In the discharge zone, the applied energy is absorbed mostly by the water vapor or the nitrogen.
In the prior art, a power supply producing 1500 watts of microwave power was necessary to assure dissociation of the gas mixture. Much of this power is wasted in recombination away from the wafer. Often the reactive ions attack the chamber, or other components, thereby introducing undesirable contamination into the process.
A problem with either nitrogen or water vapor is that the molecules diffuse everywhere, including toward the source of oxygen, thereby contaminating the source. While one could adjust the relative flows of the gases, the mixture would be determined by the diffusion problem rather than by the most effective mixture for cleaning or oxidizing a wafer.
In view of the foregoing, it is therefore an object of the invention to provide apparatus for efficiently dissociating oxygen and an additive gas in a plasma.
Another object of the invention is to provide apparatus for producing reactive ionic and neutral species from a mixture of oxygen and an additive gas, without contaminating the source of oxygen.
A further object of the invention is to provide apparatus for producing an effective amount of reactive ion species and neutral species from a mixture of oxygen and an additive gas at an applied power as low as 50 watts.